Zinc coatings have been in use for many years as an effective means of controlling the corrosion of steel. Due to the protective effect of galvanic coatings, the use of galvanized steel has increased markedly.
In recent years automobile manufacturers, designing lighter weight, corrosion resistant vehicles, have stimulated the use of galvanized steel. A drive to reduce the weight of vehicles calls for the use of high-strength, low-alloy steels with thinner wall thicknesses. The use of thinner sheet steel requires additional corrosion resistance providable through the use of galvanic coatings. Automobile manufacturers are also interested in the forming of coated sheet steel for autobody panels. Forming coated steel requires a relatively thin galvanic coating. Because of the relatively high galvanic and self corrosion of coatings of zinc per se, thick coatings have been necessary. To improve the formability of such protectively coated steel, the thick zinc coating desirably should be replaced with a thin, more corrosion resistant coating with improved galvanic protective properties.
The use of galvanic coatings is increasing in other areas also, such as reinforcing bars for concrete structures. It has been documented that corrosion of steel rebar due to halide salts within the concrete is the basis for the formation of potholes and cracked concrete. Coating the steel with a high performance galvanic coating is one proposal to alleviate this problem.
It is obvious that the increasing use of galvanized steel puts great emphasis on the performance of galvanic coatings. Although pure zinc coatings adequately protect steel, they have several drawbacks including:
(1) The widely differing electrode potentials between zinc and steel [-1.05 Vsce (Volts versus a saturated calomel reference electrode) and -0.69 Vsce respectively in aerated salt water solutions] result in excessive galvanic corrosion, where the zinc actually overprotects the steel.
(2) Hydrogen gas evolution may occur due to the large cathodic overvoltage. In some instances this may impair the mechanical properties of steel. An example of this is a steel reinforcement bar in concrete.
(3) Painted galvanized steel experiences rapid paint undercutting and delamination due to the excessively large cathodic overvoltage and corrosion product wedging.
(4) The self corrosion rate of zinc is relatively high in certain environments.
The aforementioned problems with zinc coatings are from a corrosion standpoint. Other concerns with coatings in general are: weldability, spangle (grain size), formability, paintability, and brittle intermetallic layers.
From the foregoing, it is apparent that opportunity exists for considerable improvement over the use of pure zinc coatings for corrosion protection of steel. Relatively recently some improvement has been reported in this area by using an electroplated zinc-manganese alloy (M. Sagiyama, T. Urakama, T. Adaniya and T. Hara, "Zinc-Manganese Alloy Electroplated Steel for Automotive Body", paper 86028, SAE International Congress and Exposition", Detroit, Mich., Feb. 24-28, 1986).
The present invention will teach a unique electrochemical technique for the development of galvanic coating alloys. Through its use, several new galvanic coating zinc alloys with improved corrosion resistance have been provided, as will be apparent from what follows.